If it's an Ebay auction and the seller doesn't 
reply, I'd contact ebay.  Other members have done 
that successfully when the seller has stolen an 
image.  Plus, I know of someone who watches for 
Last Unicorn fake auctions (ie they take a cel 
image and copy it to use on other junky stuff).  
So I don't see why you can contact ebay and tell 
them that you did that cel and it seems to have 
been listed by the person you sold it to as a 
production cel.  Good luck  (btw ... thanks again 
for the fantastic job you did on my FY cel ... 
are you up for another?  :D )